Coriolis-type mass flow meters operate on the principal that a fluent material passing through a flow tube, when exposed to a deflection or oscillation transverse to the direction of flow through the tubing, will react with a measurable force (the Coriolis force) on the walls of the tubing. The Coriolis reaction is generated by the fluent material moving at an instantaneously changing curvalinear path. The Coriolis reaction acts with a force directly proportional to the mass of the material in the tubing.
Sipin Pat. No. 3,355,944 discloses that a flow tube including a distortion or deflection of the flow from a straight path will substantially increase the measurability of the Coriolis reaction. Commonly assigned, co-pending U.S. application Ser. No. 912,893, filed Sept. 26, 1986 discloses a stable flow tube structure having a centralized center of gravity which is less susceptible to sensor signal contamination due to external noise or other vibrational influences. Further, the signal to noise ratio of the flow meter is increased by oscillating the flow tube at a resonant frequency which is higher than its fundamental resonance. Commonly assigned application U.S. Ser. No. 249,805, filed Sept. 27, 1988, and its continuation-in-part U.S. application Ser. No. 404,919, Filed Sept. 8, 1989, disclose that the tuning of the vibrational wave pattern, induced along the flow tube length by the oscillation of the driver, to the resonance of the Coriolis reaction will significantly increase the measurability of the Coriolis reaction and, thus, increase the ability of the flow meter to accurately determine the mass flow.
Many of the commercial Coriolis mass flow meters that are available include a curved flow tube. The curvature imparted to the tubing is the result plastic deformation, i.e. bending, of the flow tube by, typically, cold working processes. These type processes for flow tube formation create residual internal stress within the tubing material. Further, localized residual stress is created within the tubing and related flow meter elements by welding, brazing or otherwise fixing these elements onto the flow tube during assembly of the flow meter.
During normal operation, flow tubes within a Coriolis mass flow meter, whether or not having a curvature, are continuously vibrationally or rotationally oscillated at a relatively high rate of speed for extended periods of time. The non-uniformity of the flow tube meter structural elements may cause significant errors within the mass flow determination. Further, the normal operation of the flow meter may cause the flow meter structure to relax or fail due to the presence of the residual stress within the flow tube material. Variations over time in the characteristics of the flow tube and related elements may significantly effect the accuracy of the Coriolis reaction measurement and, thus, the mass flow determination.